Almost a decade ago, it was Ming, the 405-year-old clam. Then there was Jonathan, a giant tortoise who was touted as the world's oldest living creature—until questions later emerged about his identity. There are accounts of 150-year-old whales and 115-year-old reptiles. They make Lonesome George—the famous Galápagos tortoise who died last year at 100—seem relatively young in comparison.

Determining the ages of these particular animals was not overly difficult. Like all clams, Ming grew tree-like rings for every year it was alive. Jonathan and George—the tortoises—were well documented, having appeared in diaries and photographs over the years. The bowhead whale—called the longest-living mammal on Earth—was found with a century-old harpoon pin lodged inside of it.

But determining the age of other animals—particularly those born in the wild—is not such an easy task. Zoologists can take x-rays to look for growth markers in the skeletal structure. And they can easily find out how old an animal is after death, by examining certain biological markers on an autopsy.

Without x-rays or tissue samples, however, determining the age of an animal becomes a lot more difficult. Zoologists must rely on visual cues, with a little bit of guesswork thrown in. Below, a guide to what they look for in various species to determine age.

Orangutans Get Wrinkles Too

A lot of primate aging has to do with teeth, says Meredith Bastian, curator of primates and small mammals at the Philadelphia Zoo.

"If I look at teeth, I have a pretty accurate idea of how old an animal is," she says.

Specifically, Bastian is looking at a primate's molars. Worn-down molars may indicate that a primate is older—or it may indicate that a primate eats food that requires a lot of heavy-duty chewing.

She also looks at a primate's skin.

"There are indications that are very similar to humans," says Bastian. "It's very clear—you can differentiate a baby versus a juvenile versus later stages of life, by looking at wrinkly skin."

In wild male orangutans, zoologists look for something called a flange—or cheek pads—which are only visible on sexually mature dominant males. As they age and become over-the-hill orangutans, their flanges sag—much like our jowls.

"It used to be thought that only flanged males could mate because the flange helps them emit long calls to attract females," she says. "That's the dominant male strategy. Unflanged males have a sneakier strategy. They basically mate away from the flanged males and try not to get caught."

Female orangutans don't have flanges. They do wrinkle and lose bone density, much the way older humans do.

"They might have less hair if they're more stressed," adds Bastian.

One last marker? The whites of orangutan eyes. Babies have white circles around their eyes that disappear gradually over time. So if you can see whites it means that an orangutan hasn't finished weaning yet.

Another Use for Cat Hair

For dating cats, you want to start out with the hair, says Tammy Schmidt, curator of carnivores and ungulates at the Philadelphia Zoo.

Of course, you don't want to get too close to an elderly lion or tiger. But it is possible to see changes in their fur coats from a distance.

"The hair becomes duller," says Schmidt. "A cat is going to take less care and time with their fur coat [as it ages]."

There are other clues, but they may be harder to see.

"A carnivore like a lion or tiger is made to be secret and sly about what's happening to them," says Schmidt. "You need to put all of the pieces of the puzzle together."

Those pieces include things like muscle tone—animals become less toned as they age—and how the tail fits between an animal's hips.

"You look at how full the rump is," she says. "Can you see ribs? You look at how they're moving. Older animals are going to have more pronounced stepping because their eyesight is diminishing."

Only Dead Fish Admit Their Ages

The secret to aging fish is in the ear, reveals Kara Hilwig, the supervisor for the Alaska State Fish and Game lab. Hilwig was part of the team who recently aged a 200-year-old rockfish captured in Alaska.

To age the rockfish, Hilwig sliced through the animal's head and removed two tiny ear bones called otoliths. The otoliths have annual growth rings—like a clam or a tree—which can be counted to determine how old a fish is.

One caveat: The fish must be dead.

"We break the bones in half and then put them over a flame," says Hilwig. "And that's how you can discern this annual feature."

Want to age a fish without slicing its head off and digging around for ear bones? You're out of luck, she says.

"It would be very hard to determine a fish's actual age without the otolith," she says.

It's also important to make sure fish live in an environment where temperatures fluctuate. The otoliths only grow in the summertime.

"For fish down in the tropics, there's no distinct signature," she says. "So it's much harder to determine growth."